1. Field of the Invention
The present invention relates to a high-speed driver circuit for a semiconductor laser diode (hereinafter denoted as LD), in particular.
2. Related Background Art
FIG. 11 shows a convention LD driver circuit disclosed in the U.S. Pat. No. 7,145,928. The LD driver 100A shown in FIG. 11, which summarizes primary portions of the circuit disclosed therein, comprises a bias current source including a series circuit of an inductor 102 and a current source 103; and a modulation current source including a paired transistor, 104 and 105, with another current source 106. One of the paired transistors 104 is connected to the LD 101, while, the other 105 is connected to a dummy resistor 107. Respective emitters of the paired transistor are commonly connected to the current source 106.
The paired transistor, 104 and 105, receives a driving signal, which may be a differential signal, from the emitter followers. Each of emitter followers includes a series circuit of a transistor, 108 or 109, and a current source, 113 or 114, connected between two power supplies, V+ and V−. The LD driver 100A further provides a pre-driver circuit 110 in the front stage of the emitter followers. The pre-driver 110 includes a differential circuit of two transistors, 111 and 112. The input signal Vin with a complementary configuration is provided to the base of respective transistors, 111 and 112.
Another U.S. Pat. No. 7,057,459, has disclosed an integrated circuit implemented in, for instance, an operational amplifier. The circuit disclosed therein improves the slew rate by driving the output stage in push-pull mode by a complementary signal. A Japanese Patent Application published as JP-H02-023720A, has disclosed an integrated circuit using MESFETs which also improves the slew rate by providing a pulse signal to a transistor constituting the current source in the source follower stage. The pulse signal is generated by differentiating the input signal with a capacitor and has an opposite phase to that of the input signal. Then the current source may increase the current thereof only during the transition of the input signal.
When the LD 101 driven by the circuit 100A is necessary to be provided with a large modulation current, the paired transistor, 104 and 105, inevitably widens the size thereof, which is directly reflected to the increase of the input capacitor Cbe of the transistor, 104 and 105. In order to accelerate the slew rate of the circuit 100A, it is necessary to discharge the input capacitance as fast as possible. Conventionally, the emitter follower stage, 108 and 109, is put between the pre-driver circuit 110 and the paired transistor, 104 and 105, to reduce the output impedance of the pre-driver circuit 110. However, further reduction of the impedance requires the increase of the current, 113 and 114, flowing in the emitter follower, which results in the increase of the power consumption.
Moreover, the charging of the input capacitor Cbe may be carried out through the transistors, 108 and 109, in the driving mode depending on the input signal Vin, but the discharging is done only by the current source with a constant rate independent of the input signal Vin. Thus, the discharging of the input capacitor Cbe takes a longer period compared to the charging. In order to accelerate the discharging time, the current source, 113 and 114, is necessary to be further enlarged.
FIG. 12 shows another conventional LD driver circuit 100E disclosed in the prior United States patent above described, U.S. Pat. No. 7,145,928. The circuit 100B provides, in addition to the driver circuit 100A shown in FIG. 11, a paired transistors, 115 and 116, in the emitter follower stage so as to be a variable current source driven by the input signal Vin but with a phase opposite to the signal provided to the pre-driver circuit 110.
Transistors, 108 and 115, and transistors, 109 and 116, each constitutes a push-pull circuit because the signal provided for the upper transistors, 108 and 109, and that provided for the lower transistors, 115 and 116, have an opposite phase to each other. However, because the upper transistors, 108 and 109, are driven by the pre-driver circuit 110, while, the lower transistors, 115 and 116, are driven directly by the input signal, which causes a delay by the pre-driver circuit 110 in the single provided for the upper transistors, 108 and 109. Accordingly, the lower transistors, 115 and 116, receive the input signal Vin through other emitter followers including transistors, 117 and 118. Thus, in the LD driver 100B shown in FIG. 12, the power consumption thereof increases and the layout connecting two emitter followers becomes complex. Moreover, the delay in the pre-driver 110 depends on the load resistor Rc, while, that in the emitter follower inherently depends only on the transistors, 117 and 117, which makes it complex to adjust the delay between the upper and lower transistors.